Just did a 4L80E swap in my 99 GMC. Truck is 100% stock, trans is a stock 80e. I wired it up using the relay method, and changed all the needed stuff in the ECM with EFI live. Shifts in and out of gear, goes in reverse, N, and Park, it just seems to be stuck in 3rd gear when in drive. It won't up or down shift. I have codes P0740 and P0753. Any ideas as to what it could be?

Would having the trans cooler lines reversed cause this? I know that the return line has a sensor in the trans. Also, could the nsbu switch be bad, or out of adjustment? I played with it with no change. Its the switch off the 60e trans I took out.

Thanks for the help

 

Check your relay wiring, something is wrong.